A number of military and commercial applications require power amplifiers that operate in multiple, relatively narrow frequency bands with greatly differing center frequencies. For example, many radar systems operate with 10% to 20% bandwidth and center frequencies at L-band, S-band, X-band, and Ku-band. A high level of amplifier performance is required within the narrow operating bands but not outside of these bands. For example, an S-band/X-band system might operate at 3.0 to 3.5 GHz and 10.5 to 12.0 GHz, respectively. Operation below 3.0 GHz, above 12.0 GHz, and between 3.5 GHz and 10.5 GHz is not required.
To cover both of these bands with a single amplifier requires a wideband power amplifier, switched S/X-band amplifiers, or an amplifier tuned for two separate frequency bands. Wideband amplifier monolithic microwave integrated circuits do exist; however, they exhibit a significantly lower level of performance compared with amplifiers tuned to the individual bands and are difficult to scale to higher output power levels. Radio frequency switching between individual amplifiers that have been optimized for each frequency range requires a large amount of semiconductor real estate (two high-power amplifiers and two switches) and suffers reduced performance because of the insertion loss of the radio frequency switches.
Past efforts to develop amplifiers tuned at multiple bands have not been successful. Reasons for this inferior performance are that for both the wideband and multi-band power amplifiers, the circuitry typically employed has a significant amount of gain and output power that is outside of the operating bands being targeted and thus creates a potentially undesirable out-of-band noise emission, undesirable harmonic levels, and amplifier stability issues. What is needed is circuitry that is electronically reconfigurable such that an amplifier may be tuned for operation in multiple bands without creating the potentially undesirable out-of-band noise emission, undesirable harmonic levels, and amplifier stability issues.